1. Field of the Invention
The present invention relates to devices comprising cyclic plates for controlling the angle of attack of the blades of a helicopter lift rotor and it relates more particularly to the members of a device of this type which guide the axial translation and tilting movements of the cyclic plates with respect to a drive shaft of the rotor, as well as driving one of the plates with the shaft of the rotor and securing the other plate against rotation with respect to the structure of the helicopter.
2. Description of the Prior Art
It is well known that the angle of attack of the blades of a helicopter lift rotor, which is integral with a rotor shaft and is driven in rotation by this latter about an axis of rotation of the rotor, is generally controlled by a control device comprising two cyclic plates, disposed about the rotor shaft and one of which is a rotary plate, driven in rotation with the rotor shaft and mounted for rotation by means of at least one ballbearing on the other plate, which is a non rotary plate, secured against rotation about the axis of the rotor. On the non rotary plate are pivotably mounted the upper ends of pilot controls, for example control links or servo controls, which cause axial and pivoting movements of this plate on a mechanism guiding the axial translation and tilting of the non rotary plate with respect to the rotor shaft. Pitch control links, pivotably mounted by their upper end to levers controlling the angle of attack of the blades, are also pivotably mounted by their lower end to the rotary plate. The device finally comprises a mechanism for rotating the rotary plate and for securing the non rotary plate against rotation, comprising at least one member pivotably mounted on the one hand to the rotary plate and on the other to the rotor shaft and at least one member pivotably mounted on the one hand to the non rotary plate and on the other to a structural element of the helicopter. Thus, when the pilot controls are actuated which drive the non rotary plate, the movements of this latter are followed by the rotary plate, which transmits them to the levers controlling the angle of attack of the blades, through pitch control links.
Examples of control devices of this type are described more especially in French patents nos. 1 106 and 2 119 828.
In the control devices of this type which equip numerous modern helicopters, the mechanism for guiding the axial translation and tilting of the non-rotary plate with respect to the rotary shaft comprises a spherical ball joint, on which the non-rotary plate is mounted and tilts during cyclic variations of the pitch, the ball joint being itself mounted for axial sliding and moved along the rotor shaft during collective pitch variations, over a non rotary tubular guide surrounding the rotor shaft and which extends, upwardly, the housing of the main transmission box by means of which the rotor shaft is driven in rotation.
Each of the members of the mechanism rotating the rotary plate, which are pivotably connected thereto and to the rotor shaft, is an upper rotary compass, whose upper end is pivotably mounted on a compass driver connected for rotation with the rotor shaft, and whose lower end is pivotably mounted on the rotary plate, and each of the members of the mechanism for securing the non rotary plate against rotation, which are pivotably mounted to the non-rotary plate and to the structure of the helicopter, is a lower non-rotary compass, whose upper end is pivotably mounted to the non-rotary plate and whose lower end is pivotably mounted to the upper part of the housing connecting the rotor shaft to the main transmission box, these compasses being pivotably mounted by means of ball joints and self lubricating bearings.
The U.S. Pat. No. 4,326,834 has recently proposed a device for controlling the angle of attack of the blades of a rotor which comprises two annular control plates surrounding the rotor shaft, spaced axially apart from each other along this latter, and connected together at their outer radial periphery by compasses each formed from two flexible blades fixed together by their external radial end and fixed by their internal radial end, one to the upper plate and the other to the lower plate. Bearing means of biconical shape are mounted for rotation on each of the two plates and are engaged radially inwardly in peripheral grooves of corresponding shape comprised by two annular tracks. The track cooperating with the bearing means of the lower plate is integral with the lower end of a sleeve mounted for axial sliding on the rotor shaft and interlocked for rotation therewith by keying, whereas the track cooperating with the bearing means of the upper plate is fixed to the lower end of a truncated cone shaped tubular member, surrounding the sliding sleeve without contact therewith and diverging towards its upper end, which is fixed to the periphery of a flexible annular diaphragm, whose internal edge is fastened to the upper end of the sliding sleeve. The truncated cone shaped tubular member comprises radial arms, each of which is connected to the lever controlling the angle of attack of one of the blades of the rotor by means of a link. The pilot controls comprise a set of actuating levers, the pivoting of which controls the axial translational movements of the lower plate and these translational movements are transmitted on the one hand to the rotary sliding sleeve through the corresponding bearing members and by the corresponding rotary track and, on the other hand, to the truncated cone shaped tubular member by means of the flexible blade compasses, the upper plate, the bearing means thereof and the corresponding rotary ring, as well as the flexible diaphragm connecting the sliding sleeve to the truncated cone shaped tubular member and driving this latter in rotation, which allows variation of the collective pitch. The pilot controls also comprise control cables, whose sheaths secure the two plates against rotation about the rotor shaft so that the differential tractive forces exerted on the cables control the tilting, with respect to the lower plate and to the rotor shaft, of the upper plate and of the truncated cone shaped tubular member, which allows the cyclic variation of the pitch, the flexible diaphragm thus deformed tending to return the tubular member and the upper plate resiliently to a neutral position centered about the rotor shaft.
The diaphragm, which slides axially with the sleeve, is therefore only deformed so as to allow the cyclic variation of the pitch by tilting of the non rotary upper plate and of the rotary tubular member.
Because of its arrangement as well as the way it is secured by bolting its internal and external radial edges respectively to a collar of the rotor shaft and to the truncated cone shaped tubular member, it is doubtful whether the flexible annular diaphragm of such a device can be sufficiently deformed in a stable way to follow if only the angular movements of the upper plate, which are of the order of 15 to 30.degree. depending on the type of helicopter.
In addition, because of the structure of the bearing members and of the cable pilot controls for the cyclic control of the pitch, the construction of such a device must be particularly carefully done and is therefore costly if it is desired to reach the degree of accuracy of the controls required on modern helicopters.
The present invention proposes then to simplify the structure and so reduce the manufacturing and maintenance costs of control devices of the type comprising two cyclic plates one of which, which is rotary, is mounted for rotation on the other which is non rotary, without however presenting the drawback of limited performances which is attached to the control devices described in the U.S. Pat. No. 4,326,834.